Recently, efforts have been made to prepare heterophasic copolymers, such as an impact copolymer (ICP), using newly developed metallocene (MCN) catalysis technology to capitalize on the benefits such catalysts provide. Homopolymers prepared with such “single-site” catalysts often have a narrow molecular weight distribution (MWD), low extractables, and a variety of other favorable properties associated therewith, and copolymers often also have narrow composition distributions.
Unfortunately, MCN catalysts for polypropylene have generally been limited by their inability to produce isotactic polypropylene (iPP) or propylene-ethylene copolymers of high molecular weight or other desired properties. Compared to their Ziegler-Natta (ZN) catalyzed counterparts, the iPP matrix of the ICP prepared using MCN has a low porosity, and is unable to hold a sufficiently high rubber content within the iPP matrix required for toughness and impact resistance. The formation of rubber in a separate phase outside the matrix is undesirable, e.g., it can result in severe reactor fouling.
A key enabling factor to making an improved ICP with step out properties is to obtain an MCN catalyst system capable of making an iPP matrix with stiffness properties comparable to or better than conventional ZN-catalyzed iPP. To provide efficiency and flexibility for commercial polymerization processes, this catalyst system should also be capable of producing high molecular weight polymer at high catalyst activities in the presence of low or zero hydrogen concentrations. Such a catalyst system optimizes flexibility for production of different polymer grades, because hydrogen concentration can be used as a lever to reduce molecular weight in the reactor. Unfortunately, MCN catalyst systems often have low activities at low or zero hydrogen concentrations. While addition of hydrogen increases MCN catalyst activities, it can also result in polymers with lower molecular weights than what is desirable.
A further disadvantage of MCN catalysts is the requirement of large amounts of expensive activator, such as an aluminoxane, to activate the catalysts. Additionally, while homogeneous metallocene catalysts can be used in solution phase reactors, MCN catalysts generally need to be supported to be used in most other polymerization processes. Thus, while many metallocene catalysts are capable of making polyolefins with commercially desirable properties, the catalysts are often not practical or economical on an industrial scale due to the large amount of activator needed and difficulties in incorporating the catalyst and activator on a support.
It is important to find a way to incorporate the MCN and cocatalyst onto the support without losing the advantages of the homogenous MCN compound, including high catalyst activity, stereochemical control, and the ability to tailor polymer properties. Identifying the optimum properties for MCN catalyst supports is an area of significant research interest. Both the nature of the support and the method used to integrate the support and/or activator can affect the catalyst activity and the final properties of the polymer.
Although aluminoxanes are expensive, supported catalysts with higher aluminoxane loadings are desirable in some circumstances. For example, when the metallocene compound has low activity or low activation efficiency or when a multi-catalyst precursor system is used where the total catalyst precursor loadings are higher than usual, higher aluminoxane loading may be required to achieve a commercially viable catalyst activity. In polymerization processes where liquid solvent is present, such as slurry and condensed mode processes, methyl aluminoxane (MAO) is soluble in the solvent and can leach out of the silica particles. It has generally not been possible with conventional silicas, e.g., Grace 948 or 955, PQ ES 70 or ES 757, to load more than about 8 to 9 mmol Al/g of silica onto the support without leaching of MAO (and possibly catalyst) into the solvent medium. This leaching can cause fouling and fines in the reactor system and can negatively impact catalyst activity and polymer properties.
It is also important for a catalyst support to be able to retain mechanical strength under the operating conditions of the process in which it is used. Many polymerization processes take place at significantly higher than ambient temperatures and pressures. If the mechanical strength of the support is compromised, the impregnated silica particles can fragment. This can also lead to activator and catalyst leaching into the solvent medium. Additionally, polymerization can start to take place on the smaller fragmented particles, leading to agglomerates within the reactor system that can cause fouling, plugging, and other problems.
There is a need for supported MCN catalyst systems capable of polymerizing polymers with high molecular weights at low or zero hydrogen concentrations and high catalyst activities. There is a need for supports compatible with such catalyst systems that can maintain the mechanical strength necessary for a variety of polymerization process and load sufficient activator, e.g., aluminoxane, to achieve high catalyst activities.